This disclosure pertains to reflective elements (reflective mirrors) that are especially suitable for use in xe2x80x9cX-rayxe2x80x9d optical systems. By xe2x80x9cX-rayxe2x80x9d is meant not only the conventional xe2x80x9chardxe2x80x9d X-ray wavelengths of the electromagnetic spectrum but also the so-called xe2x80x9csoft X-rayxe2x80x9d (also termed xe2x80x9cextreme ultravioletxe2x80x9d or EUV) wavelengths. More specifically, the disclosure pertains to multilayer-film-coated mirrors that can be used in any of various X-ray optical systems such as X-ray microscopes, X-ray analysis equipment, and X-ray exposure (microlithography) apparatus.
As the density of active-circuit elements in microelectronic devices (e.g., integrated circuits, displays, and the like) has continued to increase with corresponding decreases in the size of active-circuit elements in such devices, the resolution limitations of optical microlithography have become apparent. To obtain better resolution of circuit elements, especially such elements having a width of 0.15 micrometer or less, increasing attention has been directed to the development of a practical xe2x80x9cnext generationxe2x80x9d microlithography technology.
A key candidate for next-generation microlithography exploits the short wavelengths of X-ray radiation. For example, EUV radiation is in the wavelength range of 11 to 14 nm, which is substantially shorter than the 157-nm wavelength representing the shortest achievable wavelength used in the deep UV radiation used in conventional optical microlithography. These shorter wavelengths in the X-ray portion of the electromagnetic spectrum offer tantalizing prospects of substantially improved pattern-element resolution (e.g., 70 nm or less) in microlithography. See, e.g., Tichenor et al., Transactions SPIE 2437:292 (1995).
The complex refractive index xe2x80x9cnxe2x80x9d of substances in the wavelength range of X-rays is expressed as n=1xe2x88x92xcex4xe2x88x92ik (wherein xcex4 and k are complex numbers). The imaginary part k of the refractive index expresses X-ray absorption. Since xcex4 and k are both considerably less than 1, the refractive index in this wavelength range is extremely close to 1. Consequently, optical elements such as conventional lenses cannot be used. Reflective optical elements, on the other hand, are practical and currently are the subject of substantial research and development effort.
From most surfaces, X-rays exhibit useful reflection only at oblique angles of incidence. In other words, the reflectivity of X-rays is extremely low at angles of incidence less than the critical angle xcex8c of total reflection, which is about 20xc2x0 at a wavelength of 10 nm. Angles greater than xcex8c exhibit total reflection. Hence, many conventional X-ray optical systems are so-called xe2x80x9coblique-incidencexe2x80x9d systems in which the X-radiation is incident at angles greater than xcex8c to the reflective surfaces in the optical systems. (The angle of incidence is the angle formed by the propagation axis of an incident beam relative to a line normal to the surface at which the propagation axis is incident.)
It has been found that multilayer-film mirrors exhibit high (albeit not total) reflectivity to X-radiation. The multilayer coating typically comprises several tens to several hundreds of layers. The layers are of materials exhibiting the highest available boundary-amplitude reflectivity. The thickness of each layer is established based on light-interference theory so as to achieve alignment of the phases of light waves reflected from the various layers. Multilayer-film mirrors are formed by alternately laminating, on a suitable substrate, a first substance of which the difference between its refractive index in the X-ray wavelength band to be used and its refractive index (n=1) in a vacuum is relatively large and a second substance of which this difference is relatively small. Conventional materials satisfying these criteria and exhibiting good performance are tungsten/carbon and molybdenum/carbon composites. These layers are usually formed by thin-film-formation techniques such as sputtering, vacuum deposition, CVD, etc.
Since multilayer-film mirrors also are capable of reflecting X-radiation at low angles of incidence (including perpendicularly incident X-radiation), these mirrors can be incorporated into X-ray optical systems exhibiting lower aberrations than exhibited by conventional oblique-incidence X-ray optical systems.
A multilayer-film mirror exhibits a wavelength dependency in which strong reflection of incident X-radiation is observed whenever Bragg""s equation is satisfied. Bragg""s equation is expressed as 2d sin(xcex8xe2x80x2)=nxcex, wherein d is the period length of the multilayer coating, xcex8xe2x80x2 is the angle of incidence measured from the incidence plane (i.e., xcfx80/2xe2x88x92xcex8), and xcex is the X-ray wavelength. Under conditions satisfying Bragg""s equation, the phases of the reflected waves are aligned with each other, thereby enhancing reflectivity of the surface. For maximal reflectivity, the variables in the equation are selected so that the equation is fulfilled.
Whenever the multilayer coating of an X-ray mirror comprises alternating layers of molybdenum (Mo) and silicon (Si), the mirror exhibits high reflectivity at the long-wavelength side of the L-absorption end of silicon (i.e., at 12.6 nm). Thus, a multilayer-film mirror exhibiting high reflectivity (over 60% at direct incidence, xcex8=0xc2x0) at xcex≈13 nm can be prepared with relative ease. As a result, Mo/Si multilayer-film mirrors are the currently most promising mirror configuration for use in reduction/projection microlithography performed using soft X-ray (EUV) radiation. This type of microlithography is termed extreme ultraviolet lithography (EUVL).
Whereas Mo/Si multilayer-film mirrors exhibit high reflectivity, as discussed above, their performance depends upon the wavelength of incident radiation and upon the angle of incidence, as indicated by Bragg""s equation. Especially with curved multilayer-film-coated mirror surfaces, the angle of incidence of an X-ray beam differs at various points on the surface of such a mirror used in an illumination-optical system or a projection-optical system of an EUVL system. The difference in incidence angle over the mirror surface can range from several degrees to several tens of degrees. Consequently, whenever a multilayer film is formed with a uniform thickness over the entire surface of the mirror substrate, differences in reflectivity at the mirror surface will be evident as a result of the differences in the angle of incidence.
FIG. 6 is a graph showing a theoretical relationship of reflectivity to the angle of incidence of a multilayer-film mirror having a period length of 69 xc3x85, a lamina count of 50 layer pairs, and an incident-light wavelength of 13.36 nm. The abscissa is angle of incidence and the ordinate is reflectivity. The solid-line curve denotes reflectivity of s-polarized light and the dotted line denotes reflectivity of non-polarized light. The period length is the total thickness of one pair of layers (i.e., in the case of a Mo/Si multilayer coating, one Mo layer with its adjacent Si layer). The ratio of the thickness of a single Mo layer to the period length is denoted xcex93; in this example xcex93 is constant at 0.35. As can be seen from FIG. 6, reflectivity changes with the angle of incidence. Reflectivity is nearly 74% at a 0xc2x0 angle of incidence, and decreases to less than 60% at a 110xc2x0 angle of incidence. This represents a greater than 10% drop in reflectivity.
A conventional countermeasure to the reflectivity drop noted above involves providing the thickness of the multilayer coating with a distribution that changes over the mirror surface in a manner serving to offset the change in reflectivity. Thus, light of a specified wavelength is reflected with high reflectivity at the various angles of incidence characteristic of various respective points on the reflective surface.
For example, FIG. 7 is a graph showing the relationship of the period length and of total film thickness (period lengthxc3x97number of layer pairs) at which reflectivity is highest for an incident xcex=13.36 nm versus the angle of incidence. The abscissa is angle of incidence, the left-hand ordinate is period length, and the right-hand ordinate is total film thickness (xcex93=0.35). As can be seen in FIG. 7, the period length and total film thickness at which reflectivity is highest are approximately 68.28 xc3x85 and 3413 xc3x85 (50 layer pairs), respectively, whenever the angle of incidence is 0xc2x0. Whenever the angle of incidence is 10xc2x0, the period length and total film thickness at which reflectivity is highest are approximately 69.31 xc3x85 and 3466 xc3x85 (50 layer pairs), respectively. Consequently, in order for reflectivity to be at its highest at the various angles of incidence, it is necessary to make the period length approximately 1 xc3x85 larger, at points at which the angle of incidence is about 10xc2x0, than at points at which the angle of incidence is about 0xc2x0. Now, Mo/Si multilayer coatings on EUV-reflective multilayer-film mirrors generally comprise 50 layer pairs. Locally increasing the period length on a multilayer coating as summarized above would create a difference of 4.7 nm in the total film thickness of the multilayer coating, which would impose a corresponding change in the surface profile of the multilayer-film mirror. Since the magnitude of this change exceeds what can be tolerated from the standpoint of wavefront aberration of light reflected from the mirror, such changes can significantly deteriorate the optical performance of an EUV optical system including such a mirror.
In view of the shortcomings of conventional multilayer-film mirrors as summarized above, the present invention provides, inter alia, multilayer-film mirrors that exhibit high reflectivity to incident X-radiation, independently of the angle of incidence and without deteriorating optical performance of the mirror. The invention also provides X-ray optical systems including such multilayer-film mirrors.
According to a first aspect of the invention, multilayer-film mirrors are provided that comprise a mirror substrate and a multilayer film on a surface of the mirror substrate. An embodiment of the multilayer film is configured so as to render the surface reflective to one or more selected wavelengths of incident X-ray light (e.g., hard X-ray light or xe2x80x9csoftxe2x80x9d X-ray light such as extreme ultraviolet (EUV) light). The multilayer film is formed of alternating superposed layers of a first and a second material arranged as multiple layer pairs superposed on the surface. The first material has a relatively large difference between its refractive index for X-ray light and its refractive index in a vacuum, and the second material has a relatively small difference between its refractive index for X-ray light and its refractive index in a vacuum. Each layer of the first material in the multilayer film has a respective thickness. In at least one of the layer pairs, a ratio (xcex93) of the thickness of the respective layer of the first material to a thickness of the layer pair has a variable distribution over at least a portion of the surface.
In the multilayer-film mirror summarized above, xcex93 can vary with changes in angle of incidence of incident radiation over at least a portion of the surface. By varying xcex93 in this manner, maximal reflectivity can be obtained at each point on the reflective surface, corresponding to the respective angle of incidence at each point. For example, xcex93 can decrease with corresponding increases in angle of incidence of incident radiation over at least a portion of the surface. Generally, the angle of incidence is greater at the perimeter of a mirror than at the center of the mirror. Hence, by decreasing F at regions where the angle of incidence is great, high reflectivity can be achieved at such regions as well as at, for example, the center of the mirror.
By way of example, the first material can comprise molybdenum, which is especially suitable for a multilayer-film mirror reflective to incident EUV light. For certain wavelengths of EUV light, the first material can include ruthenium. Also for EUV light, the second material can comprise silicon.
In another embodiment the distribution of xcex93 is stepped over at least a portion of the surface. In this configuration, each step corresponds to a respective range of angle of incidence of radiation incident to the surface.
In another embodiment the distribution of xcex93 is continuous over at least a portion of the surface. In this distribution, xcex93 varies with respective angles of incidence of radiation incident to the surface. Typically, the layer pairs have a period length. The distribution of xcex93 can be continuous over a first portion of the surface in which angle of incidence of light incident to the surface is within a respective range and the period length is constant. In a second portion of the surface outside the first portion, xcex93 can be constant while the period length is increased. Alternatively, the distribution of xcex93 can be continuous over the surface, wherein the period length changes continuously over the surface.
According to another aspect of the invention, optical systems are provided that comprise any of the various embodiments of multilayer-film mirrors such as those summarized above. The optical systems can be configured as, for example, X-ray optical systems such as EUV optical systems.
According to yet another aspect of the invention, optical elements are provided that are reflective to incident X-ray light. An embodiment of such an optical element comprises a mirror substrate and a multilayer film on a surface of the mirror substrate. The multilayer film is configured as summarized above. The optical element can be, for example, a multilayer-film mirror or a reflective reticle. One or more such optical elements can be incorporated into, for example, an X-ray optical system such as an X-ray lithography tool.
According to yet another aspect of the invention, methods are provided for producing a multilayer-film mirror. In an embodiment of such a method, a surface of a mirror substrate is configured to be a reflective surface. On the reflective surface, a multilayer-film coating is formed by applying alternating superposed layers of a first and a second material arranged as multiple layer pairs superposed on the reflective surface. The first material has a relatively large difference between its refractive index for X-ray light and its refractive index in a vacuum, and the second material having a relatively small difference between its refractive index for X-ray light and its refractive index in a vacuum. Each layer of the first material in the multilayer film has a respective thickness. In at least one of the layer pairs, a ratio (xcex93) of the thickness of the respective layer of the first material to a thickness of the layer pair has a variable distribution over at least a portion of the surface.
The multilayer-film coating can be formed such that xcex93 varies with changes in angle of incidence of incident radiation over at least a portion of the surface. Alternatively, the multilayer-film coating can be formed such that xcex93 decreases with corresponding increases in angle of incidence of incident radiation over at least a portion of the surface. Further alternatively, the multilayer-film coating can be formed such that the distribution of xcex93 is stepped over at least a portion of the surface, wherein each step corresponds to a respective range of angle of incidence of radiation incident to the surface. Yet further alternatively, the multilayer-film coating can be formed such that the distribution of xcex93 is continuous over at least a portion of the surface, wherein, in the distribution, xcex93 varies with respective angles of incidence of radiation incident to the surface.
Typically, the multilayer-film coating is formed such that the layer pairs have a period length. The distribution of xcex93 can be continuous over a first portion of the surface in which angle of incidence of light incident to the surface is within a respective range and the period length is constant. In a second portion of the surface outside the first portion, xcex93 can be constant while the period length is increased. Alternatively, the distribution of xcex93 can be continuous over the surface, wherein the period length changes continuously over the surface.
According to yet another aspect of the invention, X-ray lithography tools are provided that comprise an X-ray light source, and illumination-optical system, and a projection-optical system. The X-ray light source is situated and configured to produce an X-ray illumination beam. The illumination-optical system is situated downstream of the X-ray light source and is configured to guide the illumination beam to a reticle, so as to form a patterned beam of X-ray light reflected from the reticle. The projection-optical system is situated downstream of the reticle and is configured to guide the patterned beam from the reticle to a sensitive substrate. At least one of the illumination-optical system, the reticle, and the projection-optical system comprises a multilayer-film mirror. The multilayer-film mirror comprises a multilayer film on a surface of a mirror substrate. The multilayer film is configured so as to render the surface reflective to one or more selected wavelengths of incident X-ray light. The multilayer film is formed of alternating superposed layers of a first and a second material arranged as multiple layer pairs superposed on the surface, the first material has a relatively large difference between its refractive index for soft-X-ray light and its refractive index in a vacuum. The second material has a relatively small difference between its refractive index for soft-X-ray light and its refractive index in a vacuum. Each layer of the first material in the multilayer film has a respective thickness, and, in at least one of the layer pairs, the ratio (xcex93) of the thickness of the respective layer of the first material to a thickness of the layer pair has a variable distribution over at least a portion of the surface.
The foregoing and additional features and advantages of the invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings.